The present invention relates to the magnetic resonance imaging arts. It finds particular application in conjunction with horizontal open magnet imaging equipment of the type used to perform interventional procedures on the torso and head of human subjects and will be described with particular reference thereto. It is to be appreciated, however, that the present invention also finds application in conjunction with imaging and interventional procedures performed on other human body parts and further, in conjunction with the imaging or analysis of non-human and inanimate subjects.
In magnetic resonance imaging, dipoles are selectively aligned with a primary magnetic field. Radial frequency excitation pulses are applied to stimulate resonance in the aligned dipoles and radio frequency magnetic resonance signals are collected from the resonating dipoles. Gradient magnetic field pulses are applied to encode spatial position. When imaging the human upper torso, which includes the heart, lungs, and other moving tissue, high speed image acquisition is essential.
To promote high speed image acquisition and high resolution, high strength magnetic field gradients with high slew rates are advantageous. That is, gradients of large magnitude that can be switched on and off very quickly are desirable for improved data acquisition time and resolution. However, gradient strength varies inversely as the radius squared of the gradient coil. Stored energy, a critical factor for slew rate, varies with the fifth power of the radius of the gradient coil. Thus, for upper torso imaging using large diameter coils, the width of the patient's shoulders has been a limiting factor in prior systems. Typically, a whole body gradient magnetic field coil is about 65 cm in diameter. To improve the magnetic field gradient characteristics in the upper torso while minimizing the adverse effects of large diameter sizes, elliptical gradient coils and planar gradient coils have been utilized with some success.
To improve data acquisition speed and resolution in other parts of the human anatomy, smaller diameter gradient coils have been used, e.g., smaller diameter head or wrist coils. Typical head coils are on the order of 30 cm in diameter and wrist coils are smaller yet.
One major drawback of whole body coils, insertable coils, and local coils of the general type described above, is that they limit access to the examined patient. The gradient coils substantially surround the examined region. In order for a physician to gain access to the examined region, such as to perform a biopsy or other interventional procedure, the patient must be removed from within the gradient coil assembly. Moving the patient relative to the gradient coil assembly also moves the patient relative to the resultant image obtained from the coil assembly. The moved patient needs then to be re-registered with the diagnostic image before performing any interventional procedures.
An alternative coil design having a plurality of access ports defining holes through the gradient coil has been proposed in U.S. Pat. No. 5,304,933 to Vavrek, et al. In the Vavrek, et al. system, the local gradient coil is adapted for use with a stereotaxic device and includes an opening in the coil form positioned to minimize the destruction of the gradient fields and a mechanical slide bearing for moving the form with respect to the stereotaxic frame so that the opening may be limited in area and yet provide essentially unrestricted linear access to the patient. The windings in the neighborhood of the opening are diverted by modifying the stream function of the windings in a manner to minimize the effect of the opening on the resultant gradient field. However, one problem with the Vavrek, et al. system is that a significant level of undesirable torque is generated when the coil is operated in a uniform static magnetic field. In addition, the gradient coil layout in the Vavrek, et al. system is designed or specified using a "forward approach method," thus realizing a coil configuration having a desirable asymmetric current distribution but capable of generating only marginally acceptable levels of gradient strength and slew rate.
As shown in the Vavrek, et al. patent drawings, the access holes are quite small and therefore provide only modest access to the patient within the coil. The Vavrek, et al. coil design does not allow for building both the X and Y transverse coils on the same coil carrying member radius for improved linearity. Further, the Vavrek, et al. design does not enable a two-part coil assembly such as a system of the type having a removable split top coil to facilitate easy patient receipt and exiting from the magnetic resonance imaging system.
Another alternative approach to the prior closed gradient coil systems is suggested in U.S. Pat. No. 5,378,989 to Barber, et al. This configuration proposes a pair of axially spaced apart cylindrical gradient coil carrying members separated by a distance defining an interventional access area. A patient is disposed within the bores of the coil carrying members in axial alignment therewith. The Barber, et al. system is constructed such that large portions of current patterns disposed near the isocenter of the transverse coils are flared radially outward in order to permit an opening at the center of the magnet and along the axial direction.
Generally, such an arrangement generates a gradient set which is suitable for interventional applications but has very poor gradient field strength and slew rate performance. In addition, primarily due to the flared current pattern described above, one further disadvantage of the Barber, et al. system is very poor rise time performance. Therefore, the Barber, et al. system cannot be used for real time needle tracking or for any fast imaging techniques because of the poor gradient performance and slow rise time of the overall coil set attributable mainly to the "forward approach method" used to specify the coil conductor layout. Lastly, as with the Vavrek, et al. system described above, the Barber, et al. coil design does not allow for building both X and Y transverse coils on the same coil carrying member radius and, further, does not enable a two-part coil assembly of the type having a removable split top coil to facilitate easy patient receipt and exiting from the magnetic resonance imaging apparatus.
Morich, et al. in their U.S. Pat. No. 5,585,724 assigned to the assignee of the present invention, propose yet another alternative to the prior closed gradient closed systems wherein an axially directed interstitial gap is provided between a set of coil pairs to define an interventional access area. A coil arrangement of the type described in the Morich, et al. patent is particularly well suited for interventional applications in open magnet systems having horizontally directed main magnetic fields. However, the gradient performance of the Morich, et al. design is somewhat limited in both gradient strength and rise time.
The present invention provides a new and improved gradient magnetic coil assembly for magnetic resonance imaging which overcomes the above-referenced problems and others.